1. Field of the Invention
The present invention relates to a method of preventing polymer scale deposition in polymerization of a monomer having an ethylenic double bond, and a polymer scale preventive liquid used in the method.
2. Description of the Prior Art
In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymer deposits on the inner wall surface and so forth in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer and operators may be exposed thereto, which may cause physical disorders in the operators.
Heretofore, to prevent polymer scale deposition on the inner wall surface and so forth of a polymerization vessel, methods in which suitable substances are coated on the inner wall surface, etc. as a scale preventive agent, are known. Disclosed as scale preventive agents are, for example, a dye or pigment (Japanese Patent Publication (KOKOKU) Nos. 45-30835(1970) and 52-24953(1977)), an aromatic amine compound (Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976)) and a reaction product of a phenolic compound and an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980)).
These methods can fairly effectively prevent the deposition of polymer scale in the polymerization of a vinyl halide monomer such as vinyl chloride or a monomer mixture mainly comprised of such a monomer.
In the meantime, vinyl chloride polymers obtained by polymerization are required to have a good whiteness; for example, they are required to have an L value according to the Hunter's color difference equation, which is described below, of 70 or more.
The prior art polymer scale preventive agents generally include a great number of colored substances as exemplified typically by the dyes and pigments described in Japanese Patent Publication (KOKOKU) No. 45-30835(1970) or 52-24953(1977), the aromatic amine compounds described in Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976), and the reaction products of a phenolic compound and an aromatic aldehyde described in Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980). If the coating comprising such a colored polymer scale preventive agent is dissolved or peeled to be mixed into a manufactured polymer, causing contamination of the produced polymer, a problem may arise that the produced polymer become colored and its quality is seriously lowered. That is, according to measurement of the lightness L in the Hunter's equation described in JIS Z 8730 (1980), the L may be measured to be 65 or less.
The scale-preventing effect achieved by the prior art methods varies depending on polymerization conditions such as the kind of monomers, the type of polymerization, the kind of a polymerization catalyst, and the material of the inner wall of a polymerization vessel; it is difficult to prevent the scale deposition effectively and certainly. Specifically, when a polymerization catalyst with a strong oxidative effect such as potassium persulfate, ammonium persulfate, acetylcyclohexylsulfonyl peroxide and di-2-ethoxyethyl peroxydicarbonate is used, the scale preventive agent forming the coating may be oxidized, so that the scale preventing effect is impaired. In the case of polymerization vessels with inner wall made of stainless steel or other steels, polymer scale deposition is liable to occur as compared with vessels lined with glass. Emulsion polymerization is liable to form polymer scale deposition as compared with suspension polymerization.
Moreover, where the monomer to be polymerized is styrene, .alpha.-methylstyrene, acrylates, acrylonitrile or the like, the monomer has such a strong dissolving power against the coatings formed by the above scale prevention methods that the coating may be partly or completely dissolved away, resulting that deposition of polymer scale on the inner wall surfaces and so forth of a polymerization vessel cannot be effectively prevented. In this instance, if a stainless steel polymerization vessel as mentioned above is used, polymer scale is seriously liable to deposit on the inner wall surfaces, etc.